Memory is generally of two main categories: volatile and nonvolatile. Volatile memory loses any data as soon as the system is turned off; it requires constant power to remain viable. Most types of random access memory (RAM) fall into this category.
Non-volatile memory (NVM) is a general term for memory that retains its content when a power supply thereto is stopped. A conventional NVM device is a MOS transistor that has a source, a drain, an access or a control gate, and a floating gate. It is structurally different from a standard MOSFET in its gate, which is electrically isolated, or “floating”. Generally, there are two main types of NVM devices: floating gate based and charge-trapping based memory devices.
Floating gate based devices are typically MOS transistors that have a floating gate, in which charge is stored, and which is buried within the gate oxide, and has inter-polysilicon dielectric (IPD) beneath a control gate. More specifically, as shown in FIG. 1A, floating gate memories consist of a stacked gate MOS transistor, where the first gate is the floating gate that is buried within the gate oxide and the inter-polysilicon dielectric (IPD) beneath the control gate with IPD (which may be oxide or oxide-nitride-oxide isolating the floating gate), and the second gate is the control gate which is the external gate of the memory transistor. Floating gate devices are typically used in EPROM (Electrically Programmable Read Only Memory) and EEPROM's (Electrically Erasable and Programmable Read Only Memory).
Charge trapping devices store charge in discrete nitride traps, and are typically used in MNOS (Metal Nitride Oxide Silicon), SNOS (Silicon Nitride Oxide Semiconductor), and SONOS (Silicon Oxide Nitride Oxide Semiconductor) devices. FIG. 1B shows a typical MNOS charge-trapping memory structure.
In both the floating gate and the charge-trapping memory devices, the stored charge (which is indicative of the stored data) is retained when the power supply is removed. Causing charge to be stored in a memory device is known as programming the device. The charge needed to program the device has to be injected into the floating gate or into the nitride layer. For this purpose, two main mechanisms have been shown to be viable: Fowler-Nordheim (FN) tunneling, and channel hot-electron injection (CHE). Causing the stored charge to be removed from the floating gate or nitride layer is known as erasing the device. There are two methods of erasing or discharging: FN tunneling and emission by ultraviolet (UV) radiation.